It has been determined by us and others that several of the parvoviruses interfere with viral oncogenesis both in vivo and in vitro. The possible role of these ubiquitous agents in cancer resistance encourages us to study the phenomenon in detail. We will define the spectrum of interference between certain parvoviruses, in particular H-1 virus originally isolated from human tissue, and various viruses which are considered relevant to human malignancy. The biological interplay between H-1 virus and these transforming viruses, some of which act as "helpers," will be explored in productive and nonproductive host cell infections. The molecular basis of the interference with viral transformation, and "helper" virus replication will be sought through an understanding of the molecular biology of H-1 replication. We will seek confirmation of the observation that the transformed phenotype of human cells in vitro is permissive for H-1 replication while the normal phenotype is not. The simplicity and biochemical dependence of H-1 virus on host cell replicative machinery has proved to be more specific than at first realized; this makes the virus a valuable tool for probing many aspects of host cell function. An important aspect of parvovirus biology appears to be latency, and tissue culture and animal models for this will be examined for the mechanism of this phenomenon.